1. Heat treatment is generally overheated
If the heating temperature of the forging factory is too high or the holding time is too long at high temperature, the austenite grain coarsening is called overheating. The coarse austenite grains will lead to a decrease in the toughness of the steel, an increase in the brittle transition temperature, and an increase in the tendency of deformation cracking during quenching. The cause of overheating is that the furnace temperature meter is out of control or mixed. The superheated structure can be re-austenized under normal conditions to refine the grain after annealing, normalizing or multiple high temperature tempering.
2. Forging piece inheritance
Forgings with overheated microstructures, after reheating and quenching, although the austenite grains can be refined, sometimes large coarse fractures still appear. There are many theoretical controversies for the generation of fracture inheritance. It is generally believed that impurities such as MnS are dissolved in austenite and enriched at the crystal interface due to excessive heating temperature, and these inclusions will precipitate along the crystal interface during cooling. When impacted, it is easy to break along the coarse austenite grain boundary of the forging.
3. Genetics of large tissues
Forging is heated at a slow rate to a conventional quenching temperature, even lower, and the austenite grains are still coarse. This phenomenon is called tissue heritability. To eliminate the heritability of the coarse tissue, intermediate annealing or multiple high temperature tempering treatments may be employed.